1. Field of the Invention
The present invention relates to a fluid pressure control system for a hydraulic transmission with a lock-up clutch.
2. Description of the Prior Art
The torque converter which is now widely used for a hydraulic transmission for automobiles, comprises an impeller connected to an engine output shaft, a turbine connected to an input shaft of a gear transmission mechanism and a stator adapted to deflect the flow of fluid which returns from said turbine to said impeller in order to effect a determined torque conversion between the engine output shaft and the input shaft of the gear transmission mechanism according to a difference between the rotational speed of said output shaft and said input shaft while transmitting rotational power by means of fluid dynamics. In this torque converter, there exists inevitably a slippage between the impeller and the turbine even under the optimum operating condition where the rotational speed of the turbine has almost approached the rotational speed of the impeller, thereby lowering the power transmission efficiency when compared with the direct transmission mechanism of a mechanical clutch, resulting in an increase in the fuel consumption of the automobile. This slippage results in problems regarding economy of petroleum and exhaust gas purification. In order to solve this problem, a hydraulic transmission system equipped with a lock-up clutch has been proposed. One such lock-up clutch system is described in U.S. Pat. No. 3,255,642, said system including in parallel a hydraulic transmission, such as a torque converter, and a friction clutch, the latter being adapted to be selectively actuated by fluid pressure under the control of an oil pressure control system. The oil pressure control system effects direct transmission according to the operational condition of the automobile so that the operational advantages of both the hydraulic transmission and the direct mechanical clutch are obtained.
In order to accomplish an optimum operation of the two abovementioned types of transmissions the friction clutch must be selectively actuated depending upon the automobile operating condition in the hydraulic transmission. Also, it is necessary that the friction clutch is actuated as long as possible to avoid power loss by the torque converter in the operating condition. When torque conversion is not required, the friction clutch is temporarily disengaged when speed shifting is effected so that an abrupt change of torque due to shifting of the gear transmission is absorbed by the torque converter for effecting smooth speed shifting. For effecting such selective engagement or disengagement of the friction clutch according to the operational condition of the automobile, particularly the shifting of the gear transmission mechanism, the fluid pressure control system according to the aforementioned U.S. Patent incorporates a throttling means in a passage for supplying a servo fluid pressure to a friction engaging means of a gear transmission mechanism, whereby a pressure difference is generated on opposite sides of the throttling means when fluid is supplied to the friction engaging means through said throttling means in the shifting of speed. The pressure difference is employed for actuating a fluid pressure change-over valve which effects disengagement of the friction clutch. With the completion of the speed shifting, the flow of fluid to the friction engaging means stops, whereby the abovementioned pressure difference disappears, whereby the friction clutch is again put into its actuating condition. However, in this structure of providing a throttling means in the fluid passage extending to the friction engaging means, in view of the possibility of a large alteration in fluid temperature according to the operational condition of the transmission, such as from -30.degree. to 130.degree. C, a large alteration in the viscosity of fluid, the throttling ratio of the throttling means must be relatively large if a sufficient pressure difference is to be generated in any operating condition and. If a large throttling ratio is employed, a large time delay is suffered between the dispatch of an instruction signal for speed shifting and the actuation of the friction engaging means, resulting in giving an uncomfortable speed shift feeling to the driver or passenger. Therefore, this system involves two contradictory conditions which must be satisfied, thus making it relatively difficult to practice this system.